Heatable window with a high-pass frequency selective surface

ABSTRACT

An electrically conductive coating of an automotive heatable windshield having a frequency selective surface area that facilitates the transmission of radio frequency signals. The FSS area may be a high-pass filter such that RF signals at any polarization can pass through the glazing over a wide frequency band. The FSS area is defined by a pattern in the conductive coating such that, when the conductive coating is used to heat the windshield, electrical current flows through the FSS area to mitigate hot and cold spots.

TECHNICAL FIELD

The presently disclosed invention relates to a transparency having aconductive coating that includes frequency selective surface (FSS) thatpasses radio frequency (RF) signals in one or more frequency bands. Moreparticularly, the presently disclosed invention relates to an automotivewindshield in which a portion of an electrically heatable coatingdefines an FSS that supports radio frequency signal communicationthrough the windshield while mitigating thermal extremes and opticaldistortion in the region of the windshield in and adjacent to the FSS.

BACKGROUND OF THE INVENTION

Windows are sometimes coated with transparent metallic coatings that canbe used to electrically heat the window. By applying a DC voltage to themetallic coating, electric current is caused to flow through the coatingand across the surface of the window thereby heating the window. Suchembodiments are typically used to defrost or defog the window.

One of the consequences of using metallic coated windows is that theycan attenuate the propagation of RF signals through the window. As aresult, wireless communication into and out of buildings, vehicles, andother structures that use metallic coated windows to defrost or defogthe window can be restricted. One solution for applications in which themetallic coating interferes with the propagation of signals through thewindow has been to remove a portion of the metallic coating thatinterferes with the signals. Removal of the coating facilitates thetransmission of RF signals through the portion of the window where thecoating is removed. However, removal of the metallic coating alsonegates heating of the uncoated area allowing ice and snow to accumulateand remain on the uncoated region. In addition, it has been found thatsuch uncoated areas resulted in localized high current or low currentregions that create hot spots and cold spots in and around the uncoatedarea. Such thermal disparities result in difficulties with defrostingand deicing capabilities and also can cause localized curvatures of theglass that results in optical distortions.

One manner of addressing such problems and limitations related tometallic coatings has been to create a frequency selective surface(“FSS”) area in the coating. A FSS is a periodic pattern in the heatablecoating having either one or two dimensions (i.e. single or doubleperiodic structures) that acts as a filter for one or more bandwidths.Depending on the physical construction, materials and geometry of suchstructures, they are categorized as low-pass, high-pass, band-pass andband-stop filters. For example, as applied to automobile windshields,the FSS acts as a band-pass filter for RF communication signals.

Several FSS designs have been proposed for which laser deletion is usedto remove portions of the metallic coatings and generate an FSS area inthe coatings that allows RF signals to pass through with limitedattenuation. U.S. Pat. No. 6,356,236 B1 to Maeuser illustrates a lowloss 1.5 mm FSS grid pattern with line width of 0.3 mm. This FSS gridcan be used for an electronic toll collection (ETC) system that isoperated at 5.8 GHz. However, the FSS grid pattern creates a break inthe electrical current path that blocks even heating across the window.Therefore, such a grid pattern is not preferred for a heatablewindshield application. U.S. Pat. No. 5,867,129 to Sauer discloses FSSpatterns that incorporate vertical slots or cross-shaped slots with alength that is appropriately tuned for 5.8 GHz applications. However,those designs have not met performance requirements due to high losses.

For heated window applications, U.S. Pat. No. 6,860,081 B2 to Walton andU.S. Pat. No. 7,190,326 B2 to Voeltzel illustrate a variety of differentvertical FSS patterns that allow DC current to pass through the FSSwindow for heating purposes. The slots are oriented vertically so thatonly horizontally polarized signals can pass the FSS window with lowloss. Vehicle electronics of different applications operate at variousfrequency bands. Such electronics require good RF transmittance in allapplicable bands and for all applicable polarizations, includingvertical and circular polarizations as well as horizontal polarizations.The designs described above fail to adequately meet those requirements.U.S. Pat. No. 8,022,333 B2 to Maeuser discloses an FSS design on thethird visor area of a windshield with low loss conductive material toimprove performance. That design requires cover by black paint so theonly feasible location for the electronic device is the third visor areaof the windshield. Because rain sensor, IR camera, and night visioncamera devices already are normally located on the third visor area,space limitations and possible EMC issues make the addition of stillmore devices in that area somewhat impracticable.

As the demand for vehicle electronics continues to rapidly grow, anincreasing number of antennas also have been integrated in the vehicle.For instance, AM/FM radios, TV, cellular phone, remote keyless entry,global positioning system, electronic toll collection, and radar systemsare all included in many vehicles. Accordingly, there was a need in theprior art for a metallic coated window having an FSS that would permitthe transmission of RF signals. Furthermore, there also was a need forfacilitating RF transmission through a window while still enablingelectric current flow across the panel in a manner that does not createlocalized hot spots and cold spots around the perimeter of the FSS area.

SUMMARY OF THE INVENTION

In accordance with the presently disclosed invention, transparentwindows with a thin conductive coating have at least one frequencyselective surface that facilitates radio frequency transmission. Thedisclosed frequency selective surface can be used with heatable coatingswhile avoiding the drawbacks of the frequency selective surface designsknown in the prior art. The frequency selective surface of the presentlydisclosed invention passes RF signals over at least one frequency bandand at different polarizations with limited attenuation. The disclosedwindows can be applied to any suitable application including, but notlimited to, vehicles, buildings, or other structures in which windowsare used.

Metallic coatings that are applied to a window act as a filter thatblocks electromagnetic signals in the RF and infrared regions that areoutside that part of the electromagnetic spectrum that is visible tohumans. When voltage is applied to move electric current through thecoating, electrical resistance of the coating generates heat and causesthe coating to function as a heatable coating for de-icing or defogging.When no electrical current flows in the coating, the coating functionsas a solar control coating that reflects IR energy. A portion of themetallic coating defines an area with a frequency selective surface(“FSS”). Within the FSS area, radio frequency signals within at leastone frequency band can pass through the metal layer with little or noattenuation. The pattern of the FSS is defined by deletion lines in aportion of the conductive coating. The conductive coating is removedaccording to the pattern of the deletion lines. The deletion lines arearranged in a pattern of slots that pass signals having at least onepredetermined wavelength of the electromagnetic spectrum. The FSS areais sometimes referred to as a communication window.

In one embodiment of the presently disclosed invention, thecommunication window or FSS area includes a frequency selective surfacethat has a plurality of vertical slots that are oriented parallel toeach other within the perimeter of the communication window. For the RFsignal to resonate in the slots, the length of the vertical slots are atleast one-half of the signal wavelength that corresponds to signalshaving the lowest frequencies within the bandwidth that are to passthrough the window. Between the vertical slots, one or more sets ofhorizontal slots of predetermined length cross corresponding sets ofvertical slots within the length of the vertical slots within the set.The area of the metallic coating in which a set of vertical slotscrosses a set of horizontal slots, is referred to herein as a patcharray. Each vertical slot within a group of vertical slots is separatedby a predetermined distance such that electrical current can flow alongthe undeleted vertical coating strips between adjacent vertical slotsfor purposes of heating the underlying substrate within the perimeter ofthe FSS. The pattern of vertical slots repeats throughout thecommunication window so that the frequency selective area forms aperiodic pattern of groups of vertical slots that are separated by patcharrays.

In another embodiment of the invention, arrays of patches or elementsare defined in vertical and horizontal slots in the coating ofpredetermined sub-areas of the FSS. The patch arrays are spaced fromeach other by strips of undeleted coating and form a periodic structurewithin the perimeter of the communication window. The width and lengthof the patch array is selected to pass RF signals within a designatedfrequency band. The strips of undeleted coating enable electricalcurrent flow across the FSS area to heat the communication window whenthe window is equipped with de-icing or defogger functions.

Slots that are vertically oriented facilitate horizontally polarized RFsignals while the slots that are horizontally oriented facilitatevertically polarized signals. For patch arrays that incorporate bothvertical and horizontal slots, RF signals at any polarization can passthrough the communication window, including vertical, horizontal, andcircular polarized signals at predetermined frequencies.

In one embodiment of the FSS, the respective lengths of vertical andhorizontal slots, orientation of each element, and space between theelements are selected to facilitate RF transmission of both linearpolarized signals operated at 2.5 GHz and circularly polarized signalsoperated at 5.8 GHz for automotive electronic toll collection (ETC)applications. The same design principle is applicable to a frequencyselective surface to facilitate RF transmission of electronics operatedat other frequency bands and at other polarizations.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the disclosed invention, referenceshould now be had to the embodiments illustrated in greater detail inthe accompanying drawings and described below by way of examples of theinvention. In the drawings:

FIG. 1 is a plan view of an automotive windshield incorporating featuresof the presently disclosed invention.

FIG. 2 is a schematic view of a frequency selective surface known in theprior art.

FIG. 3 is a schematic view of part of a frequency selective surfaceaccording to a first embodiment incorporating features of the presentlydisclosed invention.

FIG. 3A is an enlarged part of the inset portion of FIG. 3 that isincluded in the circle 3A of FIG. 3.

FIG. 4 is a schematic view of part of a frequency selective surfaceaccording to a second embodiment incorporating features of the presentlydisclosed invention.

FIG. 4A is an enlarged part of the inset portion of FIG. 4 that isincluded in the circle 4A of FIG. 4.

FIG. 5 is a schematic view of part of a frequency selective surfaceaccording to a third embodiment incorporating features of the presentlydisclosed invention.

FIG. 5A is an enlarged part of the inset portion of FIG. 5 that isincluded in the circle 5A of FIG. 5.

FIG. 6 is a schematic view of part of a frequency selective surfaceaccording to a further embodiment incorporating features of thepresently disclosed invention.

FIG. 6A is an enlarged part of the inset portion of FIG. 6 that isincluded in the circle 6A of FIG. 6.

FIG. 6B is an enlarged part of the inset portion of FIG. 6 that isincluded in the circle 6B of FIG. 6.

FIG. 7 is a graphic representation of measured RF transmission loss withfrequency for a frequency selective surface according to the firstembodiment of the presently disclosed invention.

FIG. 8 is a graphic representation of measured RF transmission loss withfrequency for a frequency selective surface according to the secondembodiment of the presently disclosed invention.

FIG. 9 is a graphic representation of measured RF transmission loss withfrequency for a frequency selective surface according to the thirdembodiment of the presently disclosed invention.

FIG. 10 is a graphic representation of measured RF transmission losswith frequency for a frequency selective surface according to a furtherembodiment of the presently disclosed invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a plan view of a transparent windshield 10 thatincorporates features of the presently disclosed invention. Windshield10 is a laminated vehicle windshield formed of outer and inner glassplies 14 and 12 that are bonded together by an interposed layer 16.Preferably, layer 16 is a standard polyvinylbutyral, polyvinyl chloride,polyurethane or similar plastic material that has oppositely disposedsurfaces that are defined by an edge between those surfaces. Outer glassply 14 has an outer surface (conventionally referred to as the number 1surface) on the outside of the vehicle and an inner surface that isoppositely disposed from the outer surface and that faces one surface oflayer 16 (conventionally referred to as the number 2 surface). Innerglass ply 12 has an outer surface (conventionally referred to as thenumber 3 surface) that faces the surface of layer 16 that is oppositelydisposed from the surface that faces outer glass ply 14. Inner glass ply12 also has an inner surface that is oppositely disposed from the outersurface of inner glass ply 12 (conventionally referred to as the number4 surface). The interlayer 16 is between surfaces No. 2 and 3 of glassplies 14 and 12.

Windshield 10 further includes an electro-conductive coating or element18 which occupies the daylight opening of the transparency. Theconductive coating is incorporated into automotive window glass for useas solar shield to reduce the transmission or passage of infrared andultraviolet radiation through the window. Coating element 18 ispreferably a transparent electro-conductive coating that is applied tothe number 2 surface of the outer glass ply 14 (as shown in FIG. 1) oron number 3 surface of the inner glass ply 12, as is well known to thoseskilled in the art. Coating 18 may have single or multiplemetal-containing layers as, for example, disclosed in U.S. Pat. No.3,655,545 to Gillery et al.; U.S. Pat. No. 3,962,488 to Gillery and U.S.Pat. No. 4,898,789 to Finley. The conductive coatings have a sheetresistance of about 2.7Ω/□ for an optical transmission of about 75%.

Windshield 10 further includes a top bus bar 20 and bottom bus bar 22.Top bus bar 20 and bottom bus bar 22 are both mounted on the samesurface as coating 18 and are electrically connected to coating 18. Busbar 22 is also electrically connected to an external power source (notshown) by a lead 24. Bus bar 20 is electrically connected to a common orground line 26. Electrical current flows through metallic coating 18 inresponse to voltage applied between bus bars 22 and 20 to heat thewindshield. In other embodiments, more than two bus bars could be usedto more precisely control current flow through the electricallyconductor coating and consequently the heating pattern of theelectrically conductive coating.

Conductive coating 18 further includes a frequency selective surfacearea (“FSS area”) that defines an aperture or communication window 30that incorporates features of the presently disclosed invention. The FSSarea facilitates the transmission of RF signals through metallic coating18. The position of FSS area 30 is illustrated in FIG. 1 by dotted line.The dotted line indicates that the frequency selective surface withinthe communication window is electrically connected to coating 18.

FIG. 2 illustrates details of an FSS area known in the prior art. Asshown in FIG. 2, a set of horizontal slots 100 are arranged orthogonallywith respect to a set of vertical slots 102. Each of the vertical andhorizontal slots are an area in which the conductive coating 18 has beenremoved. When a voltage is applied between bus bars 20 and 22, anelectrical current is caused to flow generally through coating 18 toheat windshield 10. However, the pattern of prior art FSS areas preventscurrent from flowing through the FSS area and redirects current flow insignificant areas of windshield 10. Such redirected current flow causesthermal extremes in various areas of windshield 10. For example, in FFSarea 30 and in portions of windshield 10 directly above FSS area 30,window 10 remains relatively cold. In other areas, particularly at theupper corners of FSS area 30, the diverted current is particularly highcreating areas that are relatively hot. Furthermore, the sharp deviationin temperature of windshield 10 (for example, at and around the uppercorners of FSS area 30) cause local deviation in the curvature of thesurface of windshield 10. Such deviations result in optical distortionsof images viewed through those portions of the windshield 10. This isparticularly troublesome considering the location of FSS area 30 andtypical lines-of-sight through windshield 10.

Embodiments of the frequency selective surface area that are inaccordance with the presently disclosed invention are more specificallyshown in FIGS. 3, 4, 5 and 6 which are further explained below.

The FSS area in those embodiments can be generally described asincluding at least one element that has a first set of slots that areparallel to each other and a second set of slots that are parallel toeach other and that are orthogonal to the slots of said first set ofslots. Each of said slots is a strip of predetermined width andpredetermined length that is void of conductive coating 18.Communication window 30 shown in FIG. 1 is located in the bottom centerof the windshield. However, the location of communication window is notlimited to that position. Communication window 30 can be positioned atany location on the windshield. Although only one communication window30 is shown in the embodiment of the invention of FIG. 1, more than onecommunication window 30 can be provided in coating 18. The number ofcommunication windows depends on the number and types of devices thatare served by signals within the electromagnetic spectrum that must passthrough the windshield.

FIG. 3 illustrates an embodiment of communication window 30 having anFSS pattern 31 in accordance with the presently disclosed invention.Pattern 31 is comprised of vertical channels 32 and vertically orientedsquare-wave slots 34. Square-wave slots 34 are comprised of a first setof horizontal slots and a second set of vertical slots. Verticalchannels 32 and square-wave slots 34 are features wherein the conductivecoating 18 is absent such as removal by laser deletion. Verticalchannels 32 can be described as two or more vertical channels that areparallel to each other and parallel to the slots of said second set ofslots of said square-wave slots. Each of said vertical channels 32 has apredetermined width and a length that is equivalent to the height of thearea of the frequency selective surface 30. Vertically oriented portionsof square-wave slots 34 can be further described as members of thesecond set of slots each having first and second ends with the secondset of slots arranged in two columns. Members of the second set of slotsin one column connect the respective first ends of adjacent members ofthe first set of slots. Members of the second set of slots in the secondcolumn connect the respective second ends of adjacent members of thefirst set of slots. Vertical channels 32 and square-wave slots 34 arenested together in a pattern of alternating members of a linear(horizontal) array (also called a lateral array) with square-wave slots34 interposed between groups of vertical channels 32. The pattern isrepeated over the entire area of communication window 30.

FIG. 3A is an enlarged drawing that shows the detail of the verticalchannels 32 in inset 3A of FIG. 3. Vertical channels 32 areapproximately parallel to each other with d being the dimension at whichadjacent vertical channels 32 horizontally repeat being in the range of0.5 to 5 mm, and preferably in the range of 0.7 to 3 mm. The slot widthe shows the width of metallic coating 18 that is removed to generate thechannel 32. Slot width e is preferably in the range of 0.05 mm to 0.2mm. The length of the horizontal leg of square-wave slot 34 is indicatedas b. The length of the horizontal slot can be adjusted to facilitatethe passage of vertically polarized signals, with signals of aboutone-half wavelength of the lowest passing frequencies being preferred.The spacing a between consecutive members of square-wave slots 34 isadjusted to facilitate the passage of horizontally polarized signals.The larger the dimension a, the more vertical channels 32 can beaccommodated between square-wave slots 34 and, as a result, the morehorizontally polarized signals can pass the FSS window. Dimensions a, b,e, d, and the length of vertical slot c of square-wave slot 34 areadjusted to vary the characteristics of the FSS filter to meet therequirements of the supported communication device. The conductivecoating between vertically oriented channels 32 conducts electricalcurrent between adjacent channels 32 and between adjacent square-waveslots 34 of the pattern 31 in the direction from bottom bus bar 22 totop bus bar 20 to heat the window glass within the FSS area 30.

FIG. 4 illustrates another embodiment of an FSS pattern 41 in accordancewith the disclosed invention. Pattern 41 is a periodic structure that isa regular grid pattern of basic element 42. Element 42 is a patch ortwo-dimensional array of slots. A set of vertical slots 46 each havinglength h is orthogonally oriented with respect to a set of horizontalslots 44 each having length g. As shown more particularly in FIG. 4A,the dimension at which adjacent horizontal and adjacent vertical slotsrepeat is d and the width of each slot is e. The length g of horizontalslots 44 corresponds to approximately one-half the wavelength forvertically polarized signals with the lowest passing frequency. Thelength h of vertical slots 46 corresponds to approximately one-half thewavelength of horizontally polarized signals with the lowest passingfrequency. In this way, dimensions g and h primarily determine theresonant frequency band of the FSS filter. The spacings f and i are therespective horizontal and vertical dimensions between adjacent elements42. Dimensions f and i, together with d and e are adjusted to tune theFSS filter in-band response to RF transmission within a designatedfrequency band. In addition, the strips of coating 18 between eachelement 42 support electrical current flow to provide heating toportions of glass within the FSS aperture when the windshield isequipped with current flow between bus bars 20 and 22 for heatingfunctions.

FIG. 5 illustrates an example of another FSS pattern 50 in accordancewith the presently disclosed invention. The FSS structure of pattern 50is similar to the FSS structure of pattern 41 shown in FIG. 4 exceptthat groups of vertical channels 52 are added between elements 54 of thetwo-dimensional patch array 50 to improve the passage of horizontallypolarized RF signals. Another difference in FSS pattern 50 is that thedimension k, which is the length of horizontal slots in thetwo-dimensional array of elements or patches, is increased to improvethe passage of vertically polarized RF signals at a lower frequencyband. As shown in FIG. 5A, d is the horizontal dimension at which thevertical channels 52 repeat and width of each slot is e. Since verticalchannels 52 are oriented in the direction of current flow between twobus bars located at the top and bottom of the windshield, channels 52improve the heating function of FSS pattern 50 as compared to FSSpattern 41.

FIG. 6 illustrates a further example of an FSS pattern 60 in accordancewith the presently disclosed invention. FSS pattern 60 has a pattern ofgroups of vertical channels 62 and elements or patches 64. Elements orpatches 64 are two-dimensional arrays of orthogonally oriented sets ofslots that form a regular grid pattern. Groups of vertical channels 62and patches 64 are arranged in a pattern of alternating members of alinear, horizontal array. In each array, patches 64 are interposedbetween groups of vertical channels 62. FSS pattern 60 repeats acrossthe entire communication window 30. Adjacent members in each group ofvertical channels 62 repeat in the horizontal dimension at the spacing dwith e being the dimension of the width of the slots as shown in FIG. 6Awhich is an enlarged inset of FIG. 6. The patch arrays 64 have a widthof x and the distance between adjacent patch arrays is y. The length xof the horizontal slots in patches 64 is selected according to thedesired filter band. The length x is approximately one-half of thelongest wavelength corresponding to lowest frequency of verticallypolarized signals that communication window 30 is intended to pass.Dimension y is selected according to the performance requirements forthe communication window 30 to pass vertically polarized RF signals. Thesmaller the dimension y, the more patches 64 (thus more horizontal setsof slots) can be accommodated in the communication window 30 and, as aresult, more vertically polarized signals can pass the FSS window. Inaddition, groups of vertically oriented channels 62 between each patcharray 64 channel electrical current flow on the coating between eachchannel in the group of vertical channels 62 across communication window30 to heat the communication window when the windshield is equipped withheating facilities.

Without limiting the invention thereto, the following four examplesfurther illustrate particular FSS patterns of the embodiments shown inFIGS. 3, 4, 5 and 6. An electrically-conductive coating of two silverfilms that are separated by dielectric film were sputter-deposited on apiece of clear float glass having a thickness of 2.1 millimeters (mm). Alaser was used to form an FSS pattern in the coating. The coated andlased glass ply was then laminated with a second clear glass ply of thesame thickness (2.1 mm) of the first ply using an interlayer having athickness of 0.76 mm. The total thickness of the completed sample was inthe range of 4.8 to 5.1 mm. The samples were tested for RF transmittanceover a wide frequency range. Test results were recorded for bothhorizontal and vertical polarizations in the 2 GHz to 18 GHz frequencyband for one sample and in the 2 GH to 8 GHz frequency band for theother three samples. All of the data illustrated in FIGS. 7 through 10is normalized with respect to free space. The transmission loss includedadditional clear glass and interlayer losses without metallic coatingwhich is about 2 to 3 dB as compared to free space.

FIG. 7 illustrates the RF transmission characteristics of an embodimentof an FSS area in accordance with the presently disclosed inventionhaving an FSS pattern similar to that shown and described in connectionwith FIGS. 3 and 3A. The FSS area had the same length and height of279.4 mm (11″ by 11″ square). Dimension a was 6 mm; horizontal slotlength b was 12 mm; vertical channel spacing d and horizontal slotspacing c was 1 mm; and the slot width e was 0.1 mm. FIG. 7 shows thatthis design offers very good performance for horizontally polarizedsignals in the 3.5 GHz to 11.5 GHz. Also there are nulls at 2 GHz and 12GHz, respectively. In accordance with the presently disclosed invention,the frequency at which the null occurs can be shifted by varying thedimensions a and b.

FIG. 8 is a graph of the test results for an embodiment of the presentlydisclosed invention that is similar to the FSS pattern that is shown anddescribed in connection with FIGS. 4 and 4A. In this example, theaperture had the same length and height of 279.4 mm (11″ by 11″ square).Dimension f was 6 mm; horizontal slot length g was 12 mm; dimension dwas 1 mm; vertical slot length h was 24 mm; spacing d between adjacentvertical slots was 1 mm; spacing i was 4 mm; and slot width e for bothhorizontal and vertical slots was 0.1 mm. As shown and described inconnection with FIG. 8, this embodiment provides improved transmissionfor vertical polarization in the 4 GHz to 8.5 GHz frequency band. The RFtransmission characteristic of this embodiment shows that it's a highpass filter with cutoff frequency around 3.8 GHz. Overall, thetransmission characteristics are good in the pass band for RF signals atall of the polarizations including horizontal, vertical, right handcircular and left hand circular polarizations.

FIG. 9 shows the test results for an embodiment similar to the FSSpattern of FIG. 5. The aperture has an FSS area structure similar tothat of FIG. 4 except that groups of vertical channels were includedbetween elements of the patch arrays and the length k of horizontalslots was increased to 24 mm from 12 mm for the previous example. Thechannel spacing d for adjacent members of the groups of verticalchannels was 1 mm. All other dimensions remained the same as for thesample constructed in accordance with the pattern of FIG. 4. Thevertical channels were added to improve transmission performance forhorizontally polarized signals and the increased length of horizontalslots was to improve the RF transmission of vertically polarized signalsover a lower frequency band. As shown in FIG. 9, this embodimentprovides improved transmission performance for vertically polarizedsignals in the 1.8 GHz to 4.0 GHz frequency band. The RF transmissioncharacteristic of this embodiment shows that the filter provides flatresponse to signals at all of the polarizations over a wide frequencyband above 2.4 GHz. That frequency band includes signals for electronictoll collection (ETC) systems that typically operate at 2.5 GHz and 5.8GHz.

FIG. 10 shows the test results for an embodiment similar to the FSSpattern illustrated in FIG. 6. In this example, the aperture had thesame length and height of 279.4 mm (11″ by 11″ square). Dimension y was6 mm; horizontal slot length x was 18 mm; spacing d for adjacenthorizontal slots was 1 mm; spacing d for adjacent vertical slots was 1mm; and slot width e for both horizontal and vertical slots was 0.1 mm.As shown in FIG. 10, this embodiment provides an improvement in thetransmission performance for vertically and circularly polarized signalsin the 4 GHz to 8.5 GHz frequency band. The filter also provides a flatresponse to horizontally polarized signals across the frequency bandfrom 1.8 GHz to 8.5 GHz.

While the invention has been described and illustrated by reference tocertain preferred embodiments and implementations, it should beunderstood that various modifications may be adopted without departingfrom the spirit of the invention or the scope of the following claims.

What is claimed is:
 1. A heatable window comprising: a transparentsubstrate sheet having at least one major surface; a coating on saidsurface of said substrate sheet, said coating being responsive toelectrical voltage to heat said window, an area of said coating definingat least two linear arrays of elements wherein each of said elementsincludes a first set of slots that are parallel to each other and asecond set of slots that are parallel to each other with the slots insaid first set being oriented orthogonally to the slots of said secondset, each of said slots being a strip of predetermined width andpredetermined length that is void of said coating, said at least twoelements, in combination with the coating adjacent and between the slotsthereof, defining a frequency selective surface wherein the elements ofsaid frequency selective surface are aligned in at least one lineararray with a portion of the coating separating adjacent elements of saidarray, and wherein the adjacent elements of said at least one lineararray of elements are separated by a group of two or more channels, eachof said channels having a length that corresponds to the length of atleast one of said linear arrays of elements, each of said chambers alsohaving a predetermined width and being parallel to each other and alsoparallel to the slots of one of said first set of slots of said elementsor said second set of slots of said elements, said frequency selectivesurface passing signals within at least one predetermined frequencyrange and also providing thermal heating to at least a portion of saidtransparent substrate that defines said frequency selective surface,said frequency selective surface mitigating thermal extremes in areas ofsaid sheet that are adjacent to said frequency selective surface.
 2. Theheatable window of claim 1 wherein each of the channels of each saidgroup are oriented in a direction that is parallel to the slots of saidfirst set of slots in each element, said channels being spaced apartfrom each other by a predetermined, constant dimension.
 3. The heatablewindow of claim 1 wherein each of the channels of each said group areoriented in a direction that is parallel to the slots of said second setof slots in each element, said channels being spaced apart from eachother by a predetermined constant dimension.
 4. A coated window having afrequency selective surface, said window comprising: a transparentsubstrate sheet that defines a major surface; an electrically conductivecoating that covers a major surface of said substrate sheet; two or morespaced bus bars that are in electrical contact with said coating; afirst lead that contacts at least one of the bus bars and a second leadthat contacts at least a second of said bus bar to provide electricalconnections to the bus bars; and an area of said coating that defines afrequency selective surface that passes signals within at least onepredetermined frequency band, said frequency selective surfaceincluding: at least two groups of channels, each channel beingapproximately parallel to the other channels in the same group and tochannels in other groups, each of said channels having a predeterminedwidth and a length that corresponds to one dimension of the frequencyselective surface, each group of at least two groups of channels beingspaced apart from each other in a direction that is orthogonal to saidchannels by a predetermined dimension; and at least two sets of slots,the slots in each set defining a square-wave pattern, at least onesquare-wave pattern of said slots being situated between two groups ofchannels, each square-wave pattern having a predetermined width and alength that corresponds to the length of said channels, said groups ofchannels and said square-wave patterns combining to form a repeatingpattern of channels, square-wave pattern, and channels to define saidfrequency selective surface.
 5. The window as claimed in claim 4 whereinsaid slots in the channels in said groups and slots in said square-wavepattern that are parallel to the channels in said groups facilitate thepassage of horizontally polarized signals through said frequencyselective surface.
 6. The window as claimed in claim 4 wherein saidslots in said square-wave pattern that are orthogonal to the channels insaid groups facilitate the passage of vertically polarized signalsthrough said frequency selective surface.
 7. The window as claimed inclaim 4 wherein said square-wave pattern includes slots that have alength corresponding to at least one-half the wavelength of the lowestpassing signal frequency.
 8. The window as claimed in claim 4 whereinsaid frequency selective surface passes horizontally polarized RFsignals from 3.5 GHz to 11.5 GHz with low loss.
 9. The window as claimedin claim 4 wherein said electrically conductive coating is adapted toconduct electricity and wherein said frequency selective surface isoriented such that electric current flows between said channels of saidgroup of channels from the first bus bar to the second bus bar.